Conversion of nitrogen compounds from plants and animals into ammonia and ammonium is called
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Nitrogen cycle
Part II of "Matter cycles": The nitrogen cycle
Nitrogen is a part of vital organic compounds in microrganisms, such as amino acids, proteins and DNA. The gaseous form of nitrogen (N2), makes up 78% of the troposphere. One might think this means we always have plenty of nitrogen available, but unfortunately it does not work that way. Nitrogen in the gaseous form cannot be absorbed and used as a nutrient by plants and animals; it must first be converted by nitrifying bacteria, so that it can enter food chains as a part of the nitrogen cycle.
During the conversion of nitrogen cyano bacteria will first convert nitrogen into ammonia and ammonium, during the nitrogen fixation process. Plants can use ammonia as a nitrogen source.
Nitrogen fixation is carried out according to the following reaction:
N2 + 3 H2 -> 2 NH3
After ammonium fixation, the ammonia and ammonium that is formed will be transferred further, during the nitrification process. Aerobic bacteria use oxygen to convert these compounds. Nitrosomonas bacteria first convert nitrogen gas to nitrite (NO2-) and subsequently nitrobacter convert nitrite to nitrate (NO3-), a plant nutrient.
Nitrification is carried out according to the following reactions:
2 NH3 + 3O2 - > 2 NO2 + 2 H+ + 2 H2O
2 NO2- + O2 -> 2 NO3-
Plants absorb ammonium and nitrate during the assimilation process, after which they are converted into nitrogen-containing organic molecules, such as amino acids and DNA.
Animals cannot absorb nitrates directly. They receive their nutrient supplies by consuming plants or plant-consuming animals.
When nitrogen nutrients have served their purpose in plants and animals, specialized decomposing bacteria will start a process called ammonification, to convert them back into ammonia and water-soluble ammonium salts. After the nutrients are converted back into ammonia, anaerobic bacteria will convert them back into nitrogen gas, during a process called denitrification.
Denitrification is carried out according to the following reaction:
NO3- + CH2O + H+ -> ½ N2O + CO2 + 1½ H2O
Finally, nitrogen is released into the atmosphere again. The whole process starts over after release.
A schematic representation of the nitrogen cycle is shown here:

Nitrogen as a limiting factor
Although the nitrogen conversion processes often occurs and large quantities of plant nutrients are produced, nitrogen is often a limiting factor for plant growth. Water flowing across the soil causes this error.